Looks great, should be a fun build to watch...
I love the gasser style, Willys & Fords of that 39-41 era...
are you going to go with the front end high & a straight axle or low slung ??
I have an old friend Doug, that races a S/G 41 Willys P-U #777,
I haven't seen him in years now...
sorry no ideas on the fenderwell headers for Mopars, unless your going to build your own,
there's tons of header kits, Summit Racing or Jegs, like Flowmasters, Dynatech, Kooks, Hedman, Hooker, Schoenfeld, Patriots, Dougs, Moroso, etc. a bunch of places sell the tubing/bends & collectors too...
Or buy mandrel U-bends or J-Bends in the sizes you want & make all you tubes {it ain't cheap thou},
buy flanges to adapt other sets, like you said, you could modify a set for SBC {especially those that were for the Stahl bolt pattern that were bigger tubing usually} & use the BB Mopar wedge flanges & add a different style collectors...